Analytical atomic absorption spectrometry



April 28, 1970 R. c. H. MaCcoRMAc ET AL 3,508,829

ANALYTICAL ATOMIC ABSORPTION SPECTROMETRY Filed Nov. 29, 1965 2 Sheets-Sheet J April 28, 1970 R. c. H. MaccoRMAC ET AL 3,508,829

ANALYTICAL ATOMIC ABSORPTION SPECTROMETRY 2 Sheets-Sheet 2 Filed Nov. 29 1965 United States Patent O Inf. C1. oij 3/30 U.S. Cl. 356-87 3 Claims ABSTRACT F THE DISCLOSURE An apparatus for analytical atomic absorption spectrometry wherein the tube which carries the hot gas containing the material to be analyzed is heated by outside means in addition to the hot carrier gas itself, preferably an electrical winding.

This invention relates to apparatus for carrying out analytical atomic absorption spectrometry, and relates particularly to such apparatus comprising a tube along which is propagated a hot gas containing, in atomised form, the element whose concentration is to be measured. The concentration is measured as the reduction in intensity of spectral lines absorbed by the atoms carried by the gas.

Apparatus of the above kind is described, for example, in U.K. patent specication No. 975,698, and experimental results with such apparatus as described by Fuwa and Vallee in Analytical Chemistry, volume 35, 1963 at page 942 et seq. These authors quote results for concentrations of zincfcadrnium, magnesium, copper, cobalt and nickel in dilute solutions of their salts, the solutions being atomised in an air-hydrogen flame, and established lower limits of detection for these elements. Tubes made of silica, asbestos, alumina, graphite and zirconia are mentioned.

It has now been found that certain diiculties can arise in using such apparatus, particularly when the element to ybe measured forms only a very minor constituent in a mixture of two or more elements, for example when it is desired to determine a small proportion of copper in a matrix of beryllium. In such cases it has been found impracticable to use a silica tube, because after a short time the inner surface becomes coated with a deposit of the major constituent, and the transmission of light along the silica tube through the ame is impaired. In extreme cases the deposit can interact with the silica to produce devitrication, so that the tube actually disintegrates on cooling.

The variation in the light transmission in such cases can be overcome in part by using a tube having a nonspecular reflecting surface such as alumina. The mode of propagation in such tubes is thought to be different from that in a clean silica tube, in which reections from the walls are believed to play a part; in an alumina or similar tube, such reflections are much reduced.

It is found, however, that a problem remains even when using a non-reflecting tube. Diiculty is found in focussing the image of the light source on the slit of the spectrograph, and this diiculty is believed to arise as a result of the temperature gradient which exists between the hot gas in the tlame and the wall of the tube, which is at a lower temperature. This gradient is believed t0 cause a heterogeneous scattering of the light within the tube which disperses the light issuing from the tube and prevents proper focussing, thereby impairing the performance of the instrument. Lagging the tube with asbestos tape produces no significant improvement.

This defocussing problem is alleviated by the present invention, which provides apparatus for analytical atomic absorption spectrometry comprising a tube of refractory material, means for disintegrating material to be analysed and dispersing it in a hot gas which is passed along the tube, means for transmitting a spectral line along the tube and means for measuring the absorption of said line in passing along the tube, wherein means additional to said hot gas are provided for applying heating to said tube. Preferably said heating means comprises an electrical winding on the outside of said tube, which is preferably of alumina.

To enable the nature of the present invention to be more readily understood, attention is directed by way of example to the accompanying drawings, wherein:

FIGURE 1 is a semi-schematic sectional elevation of apparatus embodying the invention.

FIGURE 2 is a graph comparing the light transmission of a heated alumina tube with an unheated silica tube as a function of time when a strong salt solution is fed to the burner.

Referring to FIGURE 1, a hollow-cathode discharge tube 1 of a known kind forms a light source which emits the spectral line of the element (in the present example, copper) whose concentration is to be measured. The light passes through a lens 2 and along an alumina tube 3 which is of 8 mm. bore and 90 cm. length. At the other end of the tube a Beckmann burner 4 draws the solution containing the element to be determined (i.e. copper in this instance) from a beaker 5. The burner is fed with hydrogen and air and so aligned that the ame passes down the bore of the tube. The hot exhaust gases 6 emitted at the source end of the tube are deflected upwards by compressed air from a nozzle 7. A focussing lens 8 focusses the light which passes through tube 3 onto the slit of a spectrograph 9.

The instrument as described so far is a conventional instrument of the kind referred to in the introduction to the present specification. In accordance with the present invention the outside of tube 3 is provided with an electrical heater winding 10 comprising 15 metres of l0 SWG Kanthal A1 wire. Tube 3 and winding 10 are supported within a Pyrex tube 11 of 5 cm. bore by discs 12 formed by spirals of asbestos tape. The annular space bet-Ween the two tubes is packed with alumina 13 to provide thermal and electrical insulation, and the outside of tube 11 is lagged with asbestos tape 14 to provide further insulation and protection for personnel.

In operation winding 10 is energised from a variable auto-transformer (not shown) and tube 3 heated to an orange-red heat estimated to be about 1000 C. The temperature is not critical however, and suitable operating conditions at which good focusing is achieved are readily found 'by adjustment. The described winding dissipates about 800 watts in the operating conition.

With the flame burning and the heating current switched on, a good image of the source lamp 1 can be focused on the slit of spectrograph 9, but with the current switched off this is not possible. It is also observed that heating the tube extends the length of the flame within the tube, thereby providing a longer absorbing path for the light from the source and improving the sensitivity. The lower limit of detection for copper with the heater switched off is 0.006 rg/ml., whereas with the heater on the limit is reduced to 0.001 ,Lg/ml., a six-fold improvement.

FIGURE 2 illustrates the improvement in light transmission of a heated alumina tu'be (curve A) as compared with an unheated silica tube (curve B) when atomising an aqueous solution of lithium chloride containing 0.1 percent of lithium, i.e. a relatively concentrated solution. It will be seen that with the silica tube the transmission falls steadily with time as a result of the deposition of lithium salts on the Ivvalls, whereas the transmission of the heated tube remains substantially constant. The effect of cleaning the silica tube is to raise the transmission towards its initial value as indicated by the vertical portion of curve B.

Although described `with reference to the use of an alumina tube, the invention is applicable to other nonspecular reflecting tubes such as porcelain and also to silica tubes. In the latter case, even though only very dilute solutions can be used, heating the tube gives steadier and more consistent readings than when the tube is unheated.

We claim:

1. In an apparatus for analytical atomic absorption spectrometry of the type comprising a tube of refractory material, means including a gas burner for providing a hot carrier gas and a material to be analyzed and for disintegrating said material in a flame such that the atoms of said material are dispersed in said hot carrier gas, which hot carrier gas containing the said atoms is subsequently passed along the tube, means for transmitting a spectral line along the tube and measuring means for measuring the absorption of said line by the said atoms of the material in passing along the tube, the improvement comprising a means, additional to said hot gas, for heating the tube to improve the focusing of said spectral line at said measuring means.

2. Apparatus as claimed in claim 1 wherein said heating means comprises an electrical winding on the outside of said tube.

3. In an apparatus for analytical atomic absorption spectrometry of the type comprising a tube of alumina material, means including a gas burner for providing a hot carrier gas and a material to be analyzed and for disintegrating said material in a flame such that the atoms of said material are dispersed in said hot carrier gas, which hot carrier gas containing the said atoms is subsequently passed along the tube, means for transmitting a spectral line along the tube and measuring means for measuring the absorption of said line by the said atoms ofthe material in passing along the tube, the improvement comprising a means, additional to said hot gas, for heating the tube to improve the focusing of said spectral line at said measuring means.

References Cited UNITED STATES PATENTS 2,360,267 lO/l944 Osterheld 3138-244 3,287,557 ll/1966 Bartz Z-43.5 3,381,571 5/l968 Vallee et al.

OTHER REFERENCES Fuwa et al.: The Physical Basis of Analytical Atomic Absorption Spectrometry," Analytical Chemistry, vol. 35, No. 8, July 1963, pages 942-946.

Beck: Thermal Gas Lens Measurements, The Bell System Technical Journal, vol. XLIII, No. 4, part 2, July 1964, pages 1818-1820.

Berreman: Convective Gas Light Guides or Lens Trains for Optical Beam Transmission, J.O.S.A., vol. 55, No. 3, March 1965, pages 239-247.

Camera International, No. 4, April 1965, pages 36 and 37.

Neu: Construction of a Dual Beam Heated Infeed Cell, Journal of the Optical Society of America, vol. 43, No. 6, June 1953, pages 520 and 521.

Lvov: The Analytical Use of Atomic Absorption Spectra, Spectrochimica Acta, vol. 17, August 1961, pages 761-770.

Mislan: A Flameless Method Ifor Producing Atomic Vapour for Atomic Absorption Spectrophotornetry: Determination of Cadmium, Atomic Energy Ltd., AECL- 1941, April 1964.

RONALD L. WIBERT, Primary Examiner F. L. EVANS, Assistant Examiner U.S. C1. X.R. Z50- 118; 356-76 

